Caught at the intersection of two developing workplace trends is a little-known but critical issue for aging baby boomers: the impact of Medicare on health savings accounts.

“Medicare and its impact on HSAs is one of the least understood aspects of the health savings account program,” said Todd Berkley, president of HSA Consulting Services LLC in Minnetonka, Minn.

But it’s important because more people are working after retirement age and more employers are adding HSAs as an employee benefit.

An HSA is a tax-advantaged savings account created for individuals who are covered by high-deductible health plans to pay for medical expenses that health insurance doesn’t cover.

Contributions are made by workers and/or their employer and are limited to a maximum amount each year. For 2015, individual workers and their employer may contribute up to $3,350 to an HSA. Families and their employer may contribute up to $6,650.

HSA account holders age 55 and older may make an additional $1,000 apiece in catch-up contributions.

HSAs have several tax advantages:

The money you contribute reduces your taxable income. It grows tax-free, and withdrawals are tax-free as long as the money is used for qualified health expenses.

Unlike the funds in some other types of health care accounts, money in an HSA is portable, meaning you can take it with you if you switch employers.

The money grows from year to year if you don’t spend it; there’s no requirement to use it or lose it.

At the end of 2013, there were about 10.7 million HSAs holding $19.3 billion in assets, according to the Employee Benefit Research Institute.

“The number of employers expected to offer an HSA-eligible health plan either as a health plan option or as the only health plan option is expected to continue to increase,” said the nonpartisan research organization in Washington, D.C.

But older workers need to be on high alert once they become eligible for Medicare at age 65.

“IRS rules say that you can’t contribute to an HSA if you’re enrolled in Medicare,” said Patricia Barry, Medicare expert for AARP. “You can draw on funds already in the account, but you can’t add to them.”

There’s only one way you can get around this rule: Delay taking Medicare and Social Security.

“Postponing Medicare is the only option if people want to continue contributing to their HSAs at work,” said Barry, author of Medicare for Dummies. “If you’re eligible for Medicare but have not filed an application for either Social Security retirement benefits or Medicare, you need do nothing. As long as your employer has 20 or more employees, you have the right to postpone applying for Social Security and Medicare, and therefore [you] can continue to contribute to your HSA, until you stop working. There is no penalty for this delay.”

Larger employers are prohibited from requiring Medicare-eligible employees and their spouses to enroll in Medicare, Barry said.

“In fact, they must offer these employees exactly the same health benefits as they offer to younger workers,” she said. “And as long as these employees sign up for Medicare within eight months of their employment ending, they won’t be liable for late penalties.”

However, employers with fewer than 20 workers aren’t bound by those rules.

“They can decide for themselves whether they require people to enroll in Medicare,” Barry said. “Many do, some don’t. If they do, then Medicare becomes primary to the employer insurance, meaning Medicare settles claims first.”

In that situation, if employees don’t sign up for Medicare, the employer’s plan will not cover their bills, “so it would be like having no insurance at all,” she said.

“I therefore advise people who work for small employers and [are] approaching age 65 to be sure to find out exactly how the employer plan fits in with Medicare,” Barry said.

The only possible downside to postponing Medicare, she said, is if an employer’s high-deductible insurance plan provides prescription drug coverage that isn't “creditable,” meaning that it doesn’t pay on average as much as the standard Medicare Part D prescription drug coverage.

“If it is creditable, a person does not need Part D coverage and is allowed to postpone enrolling in a Part D drug plan without penalty until the employer drug coverage ends,” Barry said. “But if it isn’t creditable, the person has a problem. If they postpone signing up with a Part D plan, they’ll face permanent late penalties when they do finally sign up. But if they do enroll, they lose their right to contribute to their HSA.”

Employers are required to provide a written notification annually to Medicare-eligible workers on whether their prescription drug coverage is creditable. Employers also must report each year to the Centers for Medicare & Medicaid Services on the creditable coverage status of their prescription drug plan.

There’s one group that isn’t affected by the IRS rules: those covered by their spouse’s HSA at work.

The IRS rule affects only the employee who contributes to an HSA; therefore, a spouse isn’t affected and can continue to use funds from the account for medical expenses.

Deciding whether to postpone Medicare and Social Security is a financial, as well as a health, decision.

“Obviously, the coverage they can get elsewhere will need to be compared in cost and coverage to what Medicare offers,” Berkley said. “Delaying Social Security also affects cash flow of retirees but also increases the size of the Social Security check once you begin taking it.”

The group he sees most interested in that approach is made up of people who haven’t saved enough for retirement and whose employer offers an HSA-qualified health plan and 401(k).

Those workers can take advantage of opportunities that allow them to contribute more to both accounts and catch up on their savings, Berkley said.

“This gives them a few more years of power saving, while also increasing the size of the ultimate Social Security check they will receive when they do retire.”

So if contributing to an HSA is important to you, make sure you’re aware of this complex rule and see if it makes sense to postpone taking Medicare and Social Security.

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